Islamic State (formerly known as Isis) jihadists have downed an Iraqi army helicopter during fighting near the northern refinery town of Baiji, days after the extremist group issued guidelines to target such aircraft.

Military aviation officials in Baghdad said the two pilots on board were killed as the Bell 407 was hit by a shoulder-fired missile.

The attack came days after the US admitted using attack helicopters as part of an international coalition air offensive against the extremists, underscoring the risks related to the operation.

"Fixed-wing aircraft flying at 30,000ft are completely immune from the type of weapons that Islamic State fighters have, but a helicopter is not," former navy aviator Christopher Harmer, who is now an analyst at the Institute for the Study of War think-tank, said.

"When you're flying a helicopter 150ft above the ground, that helicopter can be shot with a rocket-propelled grenade or a heavy machine gun ... so yes, it is much more dangerous" .

US Defence officials said two Apache helicopters struck IS targets near northeast of Fallujah, Anbar province at the weekend.